There are significant individual differences in the way people produce the phonemes of their language. Very different but systematic variations in vocal tract configuration may produce acoustic profiles that native speakers accept as categorically equivalent. The articulatorily complex American English liquids /r/ and /l/, in particular, show several distinct types of tongue configuration. Significantly, these phonemes are among the most troublesome for children and non-native speakers, but current models of vocal tract acoustics do not account for individual variations in ways that might be useful in biofeedback speech therapy. For instance, therapists cannot currently "read" spectral information to discover which of several constrictions in the vocal tract is causing a distorted /r/. Further, articulatory feedback via ultrasound or palatography shows only a portion of the vocal tract. Previous acoustic models were limited by the lack of 3-dimensional data. Using MRI, we propose to acquire a multispeaker database of vocal tract volume and shape. These volumetric data will be used to develop comprehensive acoustic models of interspeaker differences in vocal tract configuration. Our goal is to categorize vocal tract shapes in terms of acoustic strategies for producing /r/ or /1/. We will also acquire ultrasound data for a portion of the longitudinal profile of the tongue on the same set of speakers. We will then test the ability of ultrasound curve portions to constrain categorization of full-length MM midsagittal tongue profile shapes into previously defined configuration types. These data will aid our understanding of vocal tract acoustics and articulatory variation in speech. They may be used to improve speech recognition technologies and the implementation of articulatory and acoustic biofeedback therapy techniques.